Ubuntu netbook edition UNE
The thing is lightweight desktop environments are rubbish. At least the ones I've experienced.
They lack a lot of the usability features of gnome or KDE e.g. just right clicking to open as root, or to open a terminal in that particular folder.
They seem to lack polish and look like they're from the early 90s
I popped in a OCZ 60gb ssd into my netbook and just run XP. Its fine - and actually pretty quick! Very usable.